1. Field of the Invention
This invention relates to a developing apparatus used for developing into visible form an electrostatic latent image formed on an image bearing member comprising an electrophotographic photosensitive member, an electrostatic recording dielectric member or the like, and more particularly, to a developing apparatus for developing an electrostatic latent image using a dry-type one-component nonmagnetic developer which does not contain carrier particles.
2. Description of the Related Art
In an image forming apparatus, such as a copier, an image recording apparatus, a printer, a facsimile or the like, an electrostatic latent image formed on an image bearing member comprising an electrophotographic photosensitive member, an electrostatic recording dielectric member or the like, is developed by a developing apparatus to render visible the electrostatic latent image as a toner image.
As one type of such developing apparatus, various kinds of developing apparatuses using a dry-type one component non-magnetic developer have been proposed and practically utilized. In order to increase the resolution and sharpness of an image, every developing apparatus must form a thin layer of the one-component nonmagnetic developer (hereinafter termed a toner or a nonmagnetic toner since the developer does not contain carrier particles) on a developer carrying member.
For example, as shown in U.S. Pat. No. 4,458,627, by contacting an elastic blade made of rubber or metal to a developing sleeve of a developer carrying member, and regulating by passing toner particles through a contact portion (nip portion) between the elastic blade and the developing sleeve, a thin layer of toner particles is formed on the developing sleeve, and sufficient triboelectric charges are provided on the toner particles due to friction at the contacting portion.
Of course, before regulating the thickness of the layer of nonmagnetic toner particles on the developing sleeve, the nonmagnetic toner particles must be supplied and coated on the developing sleeve.
In Japanese Patent Application Public Disclosure (Kokai) No. 58-116559 (1983), the apparatus shown in FIG. 1 is described. In this developing apparatus, an elastic roller 15 having a fur-brush structure contacting a developing sleeve 3 at a position upstream from an elastic blade 4 in the direction of rotation of the developing sleeve 3 is provided within a developing container 2 receiving a nonmagnetic toner 6, serving as a one-component developer. Particles of the toner 6 remaining on the developing sleeve 3 not consumed in development are removed by the elastic roller 15, and new particles of the toner 6 are supplied and coated on the developing sleeve 3.
In U.S. Pat. No. 4,930,438, a sponge roller, serving as an elastic roller, is described which contacts a developing sleeve and rotates in the same direction as the developing sleeve, and supplies and coats nonmagnetic toner particles on the developing sleeve.
However, when it is impossible to sufficiently remove toner particles remaining on the sleeve after passing a developing area, and to supply and coat an appropriate amount of toner particles on the sleeve by the brush roller or the sponge roller in the prior art, an excellent image cannot be obtained.
Particularly, toner particles having a small particle size (i.e., having a volume average particle size of 5-8 .mu.m) capable of forming an image having a high picture quality are not effectively removed from the sleeve.
That is, toner particles having a small particle size have larger surface areas per unit volume than toner particles having a normal particle size (having a volume average particle size of about 10-12 .mu.m). Toner particles having a small particle size also have a higher probability of friction with the developing sleeve 3. As a result, triboelectric charges supplied to toner particles having a small particle size tend to be relatively higher. Accordingly, toner particles having a small particle size have a large electrostatic adhesion force with respect to the developing sleeve, and so the removal of toner particles remaining on the developing sleeve not consumed in development tends to be insufficient.
The toner particles remaining on the developing sleeve are mixed with toner particles newly supplied onto the developing sleeve, are sent to the contact portion between the developing sleeve and the elastic blade, and are subjected to triboelectric charging with the sleeve together with the newly supplied toner particles. At that time, while the newly supplied toner particles are provided with proper electric charges by triboelectric charging, the remaining toner particles are excessively charged since they are subjected to repeated triboelectric charging. The excessively charged toner particles have a greater electrostatic adhesion force with respect to the sleeve than the toner particles provided with proper electric charges, and so those particles have difficulty in being used for development. As a result, an image obtained by development causes unevenness in density as a whole. Particularly, in a so-called non-contact-type developing apparatus, wherein the thickness of the layer of toner particles is smaller than a gap between a developing sleeve and an image bearing member at a developing area, and toner particles are flown toward the image bearing member, the above-described unevenness in density is more pronounced.
As an example, if an copying operation is performed using an original (having reflective densities of 1.5, 0.3 and 0.05 on image portions at portions S, P and Q, respectively, length l corresponding to one circumference of the developing sleeve, and direction G of development), as shown in FIG. 2A, when the removal of toner particles remaining on the developing sleeve not consumed in development by the elastic roller is insufficient, a copied image of the original becomes as shown in FIG. 2B.
That is, while toner particles on a region corresponding to portion S on the developing sleeve are consumed by the development of portion Sa, which is an image corresponding to portion S of the original, at the first revolution of the developing sleeve, toner particles remain on the sleeve without being consumed on the other regions. The toner particles on the other regions are not sufficiently removed from the sleeve by the elastic roller. Hence, in a developed image at the second revolution of the sleeve, portion Pb has a density corresponding to the density of portion P of the original, but the density of portion Pa other than portion Pb is low. That is, portion Pb looks like a ghost of portion Sa. Accordingly, the above-described phenomenon will be hereinafter termed a ghost phenomenon. Such a ghost phenomenon cannot be prevented even by applying an oscillating bias voltage for increasing the development efficiency to the sleeve.
In order to solve the above-described problems, such as uneven density and the like, it is necessary to increase the removing capability of the elastic roller for toner particles remaining on the developing sleeve.
On the other hand, if, for example, an entirely black image is developed when the supply of new toner particles onto the developing sleeve by the elastic roller is insufficient, a phenomenon occurs wherein the density of image portions developed at the second or later revolution of the developing sleeve becomes smaller than the density of image portions developed at the first revolution of the developing sleeve. FIG. 3 shows how the densitites of such images look like.
In FIG. 3, copied images are obtained by being developed in the direction of arrow G, and length l corresponds to one circumference of the developing sleeve. A portion corresponding to the first revolution of the developing sleeve has the greatest density, and densities become smaller according to the order after the second revolution. This phenomenon is caused by the fact that the amount of toner particles supplied by the elastic roller after the second revolution is smaller than the amount of toner particles consumed in development at the first revolution of the developing sleeve. This phenomenon will be hereinafter termed a density diminishing phenomenon.
Particularly, since the above-described toner particles having a small particle size have poorer flowability than toner particles having a normal particle size, the supply of the toner particles having a small particle size to the developing sleeve tends to be insufficient. Accordingly, sufficient attention must also be paid to the toner supply capability of the elastic roller.